Just tried out the game today, and I would say that I'm quite pleased with the quality of the game. That said, it's possible to create (or at least modify) levels to extend the game. I haven't thoroughly gone through everything, but here's a basic outline of the level format.
The level data are stored in the Skybox and Missions folders. The Skybox folder defines the level map (the cubemap, planets, various objects, clouds, etc.). The Missions folder contain enemy sequence for each level. I haven't gone through the Skybox folder in any detail, so I won't be covering that yet. What I would cover is the Missions folder.
Inside the Missions folder, there are files following the naming convention of missionx.txt, where the first x is the level number. It is a basic text file defining the player and the enemies.
PlanetPos: where the planet is in the map. This is used for the AI for homing in its meteorites and other planet-attacking enemies.
Weapons: Player's weapons. The first number is the type of laser/plasma, and the second is rockets. A -1 means that the weapon is not enabled.
Force: Not sure yet
Indicators: haven't tested. They could correspond to the lasers and rockets, though
CamLookAt: Probably where the player is located
CityRadius: Range of the city, probably for meteorite AI
BonusGlobalPossibility: How often the player would be offered a powerup
BonusCount: Types of bonuses defined
Bonus: Defines a type of bonus and the probability of it appearing. There are many types of powerups. Look at mission40.txt for the various different ones.
FlagshipCount: Number of enemy waves
FlagshipType: Wave type. No idea what it's really supposed to do. Will test out later
FlagshipPos: Where the enemies enter from
AppearanceTime: probably how long it takes for all the enemies of the wave to appear, though it could also mean how long for each of the enemies to appear.
FighterWaves: How many subwaves of enemies there are
FighterCount: How many enemies in each subwave
Fighter: type of enemy (repeat line until there are the same numbers of Fighter lines as the number in FighterCount
DestroyerWaves: Same format as the set for FighterWaves, except these probably target the planet
...
Repeat this block the same number of times as FlagshipCount.
That's the basic structure of the level file. Try making a level on your own now. I'm not sure if it's possible to add any levels, but you can certainly overwrite one of the level slots. As soon as I figure out more of the format and the Skybox files, I'll post some custom levels here. Have fun.